Diaryl carbonates, such as diphenyl carbonate, are an important reactant in the production of polycarbonate resins. As the uses to which polycarbonates are put have increased, the safe and efficient production of diaryl carbonates has become of greater significance. Early processes for the production of diaryl carbonates utilized phosgene as a reagent. The toxicity of phosgene, however, prompted the development of a non-phosgene process. The non-phosgene process is well-known in the art and is described in U.S. patent application Ser. Nos. 4,410,464, 5,344,954, 6,784,277, 7,141,641, and 7,288,668, which are incorporated herein by reference.
As shown schematically in FIG. 1, the non-phosgene process involves three reaction steps. First, a dialkyl carbonate (e.g. dimethyl carbonate) and water are formed by reacting an alkanol (e.g. methanol) with oxygen and carbon monoxide in the presence of a catalyst. The dialkyl carbonate is separated from water and residual reaction components including carbon monoxide, oxygen, and alkanol. In a second reaction step, the dialkyl carbonate reacts with an aromatic alcohol (e.g. phenol) to produce an arylalkyl carbonate (e.g. phenylmethyl carbonate) and an alkanol (e.g. methanol). Then, in the third reaction step, two molecules of an arylalkyl carbonate undergo a disproportionation reaction to produce one molecule of diaryl carbonate (e.g. diphenyl carbonate) and one molecule of dialkyl carbonate. The diaryl carbonate is separated from the byproducts and residual reaction components (i.e. phenol, methanol, and dimethyl carbonate) to produce a product diaryl carbonate stream.
To aid in the efficiency of producing diaryl carbonates, it would be useful to recycle the byproducts and residual reaction components (i.e. aromatic alcohol, alkanol, and dialkyl carbonate). Possible uses of these components include the reuse of the dialkyl carbonate in the formation reaction to produce diaryl carbonate. Furthermore, residual alkanol can be reacted to form/reform dialkyl carbonate.
Cost-efficient recycling of the byproduct stream has however proven to be difficult to achieve. Process instabilities occurring in the diaryl carbonate production facility lead to inconsistent chemicals and chemical concentrations in the byproduct line. For example, if there is a water leak (e.g. from heat transfer equipment) in the diaryl carbonate production facility, the byproduct line would contain water. Water in a dialkyl carbonate formation reactor hinders the formation reaction of alkanol into dialkyl carbonate. Furthermore, a water leak in the diaryl carbonate production facility may cause separation problems leading to an increase in byproducts and residual reaction components in the byproduct line. Diaryl carbonate formation reaction byproducts or residual reaction components including aromatic alcohol (e.g. phenol) and its aromatic ether byproduct (e.g. anisole) can lead to reactivity or separation problems or to the formation of chlorinated aromatics (e.g. di- and tri-chlorophenols and chloroanisols) which can degrade in separation equipment of a dialkyl carbonate plant and cause fouling in piping and process equipment. When this happens chlorophenols and chloroanisoles can be formed in the dialkyl carbonate reactors and can thermally decompose in later stages of the process forming a coke-type residue that can clog transfer lines, reactors, separation equipment, and heat transfer equipment.
A plant using a byproduct line from a diaryl carbonate formation reaction would be required to constantly determine what is present in the byproduct line, determine the concentration of its components, and determine a strategy for separating and using the desired components from the balance of the byproduct line. This has proven to be cost inefficient in that significant additional labor, energy, and equipment costs would be required to recycle the byproduct line. Due to this cost inefficiency and no matter what the good intentions of the company toward the environment are, the byproduct line and its contents are often discarded.
It would be extremely beneficial to find a process where the components in the byproduct line from the diaryl carbonate production plant could be efficiently recycled.